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A 2-dimensional predator-prey model with five parameters is investigated, adapted from the Volterra–Lotka system by a non-monotonic response function. A description of the various domains of structural stability and their bifurcations is given. The bifurcation structure is reduced to four organising centres of codimension 3. Research is initiated on(More)
We define a notion of Ricci curvature in metric spaces equipped with a measure or a random walk. For this we use a local contraction coefficient of the random walk acting on the space of probability measures equipped with a transportation distance. This notions allows to generalize several classical theorems associated with positive Ricci curvature, such as(More)
Let Γ be a finitely generated, amenable group. We prove that if Γ has a nontrivial, orientation-preserving action on the real line, then Γ has an infinite, cyclic quotient. (The converse is obvious.) This implies that if Γ has a faithful action on the circle, then some finite-index subgroup of Γ has the property that all of its nontrivial finitely generated(More)
The trajectories of a vector field in 3-space can be very entangled; the flow can swirl, spiral, create vortices etc. Periodic orbits define knots whose topology can sometimes be very complicated. In this talk, I will survey some advances in the qualitative and quantitative description of this kind of phenomenon. The first part will be devoted to vorticity,(More)
Géométrie différentielle/Physique mathématique Feuilletages des espaces temps globalement hyperboliques par des hypersurfaces à courbure moyenne constante Foliations of globally hyperbolic spacetimes by CMC hypersurfaces Résumé Nous annonçons le résultat suivant : toute variété lorentzienne de dimension 3, globalement hyperbolique maximale, à courbure(More)
In this paper we study foliations F on compact manifolds M , of real codimen-sion 2, with a transversal holomorphic structure. We construct a decomposition of M into dynamically defined components, similar to the Fatou/Julia sets for iteration of rational functions, or the region of discontinuity/limit set partition for Kleinian groups in P SL(2, C). All(More)
  • Esmaa Bekkara, Charles Frances, Abdelghani Zeghib, Étienne Ghys
  • 2006
Degenerate Riemannian metrics exist naturally in various contexts. Unfortunately, their study stops to the 'admission of failure' that they are too poor, for instance, to generate a coherent intrinsic or extrinsic differential geometry, e.g. a kind of Levi-Civita connection. In this first text, we start the investigation of rigidity aspects of these(More)